Opioid Receptors

Opioid receptors are currently classified into three groups, κ, δ and μ receptors. Subtypes within each group have been suggested including μ1, μ2, δ1, δ2, κ1, κ2, and κ3. An opioid-like receptor with a high degree of homology to μ, δ and κ-opioid receptors has been cloned and termed the orphan opioid receptor (NOP, ORL1). This receptor has a distinct pharmacological profile from the classical opioid receptors and an endogenous ligand, nociceptin (orphanin FQ) has been identified.